A plasma display panel (hereinafter referred to simply as a PDP) allows achieving a high definition display and a large-size screen, so that television receivers (TV) with a large screen having as large as 65 inches diagonal length can be commercialized by using the PDP. In recent years, use of the PDP in high-definition TVs, which need more than doubled scanning lines than conventional NTSC method, has progressed and the PDP free from lead (Pb) has been required in order to contribute to environment protection.
The PDP is basically formed of a front panel and a rear panel. The front panel comprises the following elements:                a glass substrate made of sodium-borosilicate-based float glass;        display electrodes, formed of striped transparent electrodes and bus electrodes, formed on a principal surface of the glass substrate,        a dielectric layer covering the display electrodes and working as a capacitor; and        a protective layer made of magnesium oxide (MgO) and formed on the dielectric layer.The rear panel comprises the following elements:        a glass substrate;        striped address electrodes formed on a principal surface of the glass substrate,        a primary dielectric layer covering the address electrodes;        barrier ribs formed on the primary dielectric layer; and        phosphor layers formed between the respective barrier ribs and emitting light in red, green, and blue respectively.        
The front panel confronts the rear panel such that its surface mounted with the electrodes faces a surface mounted with the electrodes of the rear panel, and peripheries of both the panels are sealed in an airtight manner to form a discharge space therebetween, and the discharge space is partitioned by the barrier ribs. The discharge space is filled with discharge gas of Ne and Xe at a pressure ranging from 400 Torr to 600 Torr. The PDP allows displaying a color video through this method: Voltages of video signals are selectively applied to the display electrodes for discharging, thereby producing ultra-violet rays, which excite the respective phosphor layers, so that colors in red, green, and blue are emitted, thereby achieving the display of a color video.
The protective layer formed on the dielectric layer of the front panel of the foregoing PDP protects the dielectric layer from ion impact caused by the discharge, and emits primary electrons for generating address discharges. The protection of the dielectric layer from the ion impact plays an important role for preventing a discharge voltage from rising, and the emission of primary electrons for generating the address discharges also plays an important role for eliminating a miss in the address discharges because the miss causes flickers on videos.
To reduce the flickers on videos, the number of primary electrons emitted from the protective layer should be increased. For this purpose, impurities are added to MgO or particles of MgO are formed on the protective layer made of MgO. These instances are disclosed in, e.g. Patent Documents 1, 2, 3.
In recent years, the number of high-definition TV receivers has increased, which requires the PDP to be manufactured at a lower cost, to consume a lower power, and to be a full HD (high-definition, 1920×1080 pixels, and progressive display) with a higher brightness. The characteristics of emitting electrons from the protective layer determine the picture quality, so that it is vital for controlling the electron emission characteristics.
A protective layer with a mixture of impurities has been tested whether or not it can improve the electron-emission characteristics; however, when the characteristics can be improved, electric charges are stored on the surface of the protective layer. If the stored electric charges are used as a memory function, the number of electric charges decreases greatly with time, i.e. an attenuation rate becomes greater. To overcome this greater attenuation rate, a measure is needed such as increment in an applied voltage. The protective layer thus should have two contradictory characteristics, i.e. one is a high emission of electrons, and the other one is a smaller attenuation rate for a memory function, namely, a high retention of electric charges.
MgO particles are formed on he protective layer made of MgO for satisfying the foregoing characteristics contradictory to each other. If no MgO particles are available on the protective layer, needle crystals of the protective layer material are grown uniformly in discharge cells by a discharge, and the needle crystals can prevent the protective layer from being dug by sputtering. However, the formation of MgO particles on the protective layer made of MgO has the needle crystals grow selectively on MgO particles, so that the sputtering to a region, having no needle crystals, of the protective layer is promoted, and the service life of the PDP is thus obliged to be shortened.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2002-260535    Patent Document 2: Unexamined Japanese Patent Application Publication No. H11-339665    Patent Document 3: Unexamined Japanese Patent Application Publication No. 2006-59779